In conventional practice, torque wrenches are provided with a hinge, a pivot and a plunger aligned in that order within a tubular lever arm. The hinge fits loosely within the tubular lever arm and is rotatably mounted therein near one end of the lever arm about a mounting pin extending normal to the plane of application of torque. The hinge is connected to engage and rotate a workpiece at a connection proximate to the mounting pin. A coil spring, compressed to an adjustable degree, exerts a predetermined force on the plunger which transmits this force toward the hinge through the pivot which is interposed therebetween. The axial force of the coil spring maintains the plunger, pivot and hinge all in axial alignment within the tubular lever arm until a predetermined limit of torque is applied to the lever arm. This limit is determined by the degree of compression of the coil spring which may be altered by a calibrated adjustment mechanism. When the maximum torque limit is reached, the workpiece transmits sufficient resistance to the hinge, opposing further rotation, so that the continued application of torque ceases to rotate the hinge with the lever arm, but instead causes an angular rotation of the pivot within the lever arm, allowing the lever arm to rotate until the hinge contacts its interior wall. The shifting of the pivot creates a momentary reduction of resistance to the applied torque which can be felt by the operator, thus providing a signal that the upper limit of torque has been reached.
In conventional torque wrenches the pivot is typically formed in the shape of a rectangular prism. Recesses in both the hinge and plunger accommodate the pivot and allow it to tip to accommodate the shifting of the lever arm relative to the hinge bringing the hinge in contact with the interior wall of the encasing lever arm once the predetermined limit of torque has been reached. The recesses in the opposing facing surfaces of the plunger and the hinge are centrally located along the torque wrench lever arm axis. The recesses take the form of centered depressions formed with a flat floor parallel to and recessed from the respective surface in which the depressions are created. The walls of the depressions are inclined outwardly in the surfaces in which they are formed in the direction in which torque is applied. Normal to this direction, the walls of the depression are formed at right angles to the opposing hinge and plunger surfaces. This construction permits the pivot to tip when a maximum allowable torque is applied, but prevents movement of the pivot normal to the direction of application of torque. A peripheral surface area of the faces of both the plunger and hinge members lies between the edges of the centralized depressions therein and the boundary perimeters of the plunger and hinge members. The reason that centralized depressions are employed in both the hinge and plunger members of a conventional torque wrench is to ensure that the pivot is always trapped in the depressions to lie along the longitudinal axis of the lever arm of the torque wrench.
The manufacture of hinge and plunger members with central depressions is extremely difficult. The machining required to create central depressions of square cross section in the opposing faces of the plunger and hinge members requires an imprintation process to deform the metal of the plunger and hinge by striking those members sharply and with considerable force to deform the metal. Since both the plunger and hinge are formed of hardened steel, extremely heavy and expensive impact machinery is required to achieve the necessary indentations in the plunger and hinge faces. Even so, with conventional impact hammers, a depression of only about 0.030 inches in depth is achieved in the faces of the plunger and hinge members. This creates a very narrow bearing surface against which torsional forces are transmitted through the pivot. As a result, pressure on the pivot, especially at the corners is extremely high and the pivots of conventional torque wrenches do tend to deteriorate with use, and hence have a shortened life.
It is an object of the present invention to provide an improved torque wrench which is easy to manufacture and which has a greater useful life than conventional torque wrenches. This is achieved by milling a channel across the entire width of the opposing faces of the plunger and hinge members. The channels replace the centralized depressions which would otherwise be necessary, and thereby eliminate the imprintation process entirely in the manufacture of torque wrenches. The pivot is retained centralized along the torque wrench axis and is prevented from sliding laterally within the channels by a pivot retaining mechanism, which may take one of several forms. A disk with a rectangular aperture therein having an outer diameter to snugly fit within the hollow lever arm of the torque wrench ensures that the pivot remains centralized along the torque wrench axis. Alternatively parallel pins may be anchored to the plunger or hinge member to extend transversely across the channel formed therein. The pins are separated by a distance sufficient to accommodate the torque wrench pivot. Force transmitted to the pivot may cause the pivot to bear against the transverse pins, which serve to confine the pivot in the center of the channel to maintain it in a position centered on the torque wrench axis. Yet a further form of the invention permits an axial guide rod to extend longitudinally through the plunger to engage and restrain lateral movement of the pivot.
With all of the foregoing embodiments, the pivot is laterally immobilized against movement in the direction normal to the plane of application of torque, but the pivot is allowed sufficient latitude in the plane of application of torque so that it can tip once the predetermined upper limit of force has been reached.
Another object of the invention is to reduce the rattling noise of the pivot within the torque wrench. In conventional torque wrenches, the pivot is entrapped loosely between the two centralized depressions in facing surfaces of the hinge and the plunger. When the wrench is not subjected to torsion, the pivot rattles within its confines as the wrench is moved about. Such rattling is undesirable, as it gives the impression to the user that the tool is worn or somehow defective. In the embodiment of the present invention in which a disk with a central rectangular aperture is provided as the pivot retaining mechanism, the aperture in the disk may be formed with a punch press. This creates fenders or shoulders at the edges of the aperture which fit snugly against the pivot, yet which are cantilevered sufficiently to defect so as not to inhibit the pivot in its angular rotation relative to the hinge and the plunger. By holding the pivot immobile even when the torque wrench is not in use, this embodiment of the invention prevents the pivot from rattling within the torque wrench, thereby eliminating a source of concern to the user.
Yet another embodiment of the invention reduces wear on the plunger. During the course of use, the plunger reciprocates repeatedly within the tubular lever arm housing the hinge, the pivot and the plunger in order to accommodate the movement of the pivot. In conventional torque wrenches, the plunger is manufactured of steel as is the tubular housing. Thus, a recurring wearing of steel against steel results with the possibility that the housing will become gouged or deformed by the plunger over a period of time. Such deformation causes the torque wrench to misfunction and for the plunger to fail to reciprocate when the predetermined limit of torque has been reached should it catch in a crease or area of scoring in the wall of the housing. According to one of the embodiments of the present invention, however, the plunger is formed with a central axial guide rod which terminates near the hinge in a collar or sleeve of a low friction material, such as brass or scintered bronze. By creating the plunger of such a low friction material, scoring and deformation of the interior walls of the tubular lever arm is avoided, thereby preventing catching or other irregularities of movement of the plunger within the lever arm.